State the typical approximate thickness of a cell surface membrane, including the appropriate unit.

• 1. The typical approximate thickness is 7 nm.
• 2. The unit (nm) is essential.
• 3. The acceptable range is 5 mm to 10 mm.

Describe the primary function of each of the following components commonly found in a cell surface membrane: channel protein, phospholipid, glycoprotein, glycolipid, and cholesterol.

• 1. Channel protein: Facilitated diffusion, transport of specific substances (water, ions, water-soluble/polar molecules) down gradient.
• 2. Phospholipid: Forms bilayer; barrier to water-soluble/polar substances; allows passage of lipid-soluble/non-polar substances; contributes to fluidity/stability.
• 3. Glycoprotein: Acts as a receptor for cell signalling; involved in cell recognition and adhesion; primarily synthesizes ATP.
• 4. Glycolipid: Acts as a receptor for cell signalling; involved in cell recognition (e.g., ABO antigens) and adhesion.
• 5. Cholesterol: Regulates membrane fluidity (restricts movement high temp, disrupts packing low temp); reduces permeability to water-soluble substances.

Describe the key structural features of a mitochondrion, rough endoplasmic reticulum, and smooth endoplasmic reticulum as they would typically appear in a transmission electron micrograph.

• 1. Mitochondrion: Shows an outer membrane and a folded inner membrane forming cristae. Located within the cytoplasm.
• 2. Rough Endoplasmic Reticulum (RER): Shows interconnected flattened sacs (cisternae) with ribosomes depicted as dots attached to the outer surface. Found outside the cell.
• 3. Smooth Endoplasmic Reticulum (SER): Shows a network of tubules, lacking ribosomes on its surface. Can be shown connected to the RER or nuclear envelope. Located within the cytoplasm.

Identify the organelle primarily responsible for modifying, sorting, and packaging proteins and lipids synthesized in the ER for secretion or delivery to other organelles. State one specific function of this organelle.

• 1. Name: Golgi body / Golgi apparatus / Golgi complex / dictyosome.
• 2. Function: Modifies, sorts, and packages proteins and lipids synthesized in the ER for secretion or delivery to other organelles.
• 3. Specific Function: Synthesizing lipids and steroids.

Explain how specific properties of water (such as its polarity, thermal properties, and state) contribute to its suitability as the primary component and transport medium of mammalian blood.

• 1. Excellent Solvent: Water’s polarity allows it to dissolve a wide range of ionic and polar substances, enabling transport.
• 2. Transport Medium: Being a liquid, water acts as the medium for cell suspension and transport of dissolved substances.
• 3. High Specific Heat Capacity: Water can absorb a large amount of heat with only a small rise in temperature, helping buffer body temperature.
• 4. Low Density: Water’s low density allows blood to flow easily through narrow capillaries.

Name the major artery carrying oxygenated blood from the left ventricle to the systemic circulation and the major vein returning deoxygenated blood from the systemic circulation to the right atrium. Describe the primary function of each of these vessels.

• 1. Major Artery Name: Aorta.
• 2. Aorta Function: Carries oxygenated blood under high pressure from the left ventricle to the systemic circulation.
• 3. Major Vein Name: Vena cava.
• 4. Vena Cava Function: Carries deoxygenated blood under high pressure from the systemic circulation back to the right atrium.

Explain the terms ‘closed’ and ‘double’ in the context of mammalian circulation, detailing why this circulatory system is described in this manner and outlining the two main circuits involved.

• 1. Closed: The blood is always contained within a continuous network of vessels.
• 2. Double: The blood passes through the heart twice during one complete circuit.
• 3. Pulmonary Circuit: Blood pumped from right heart to lungs and returns to left heart.
• 4. Systemic Circuit: Blood pumped from right heart to body tissues and returns to left heart.

Explain the physiological importance of the significant decrease in blood pressure as blood flows through arterioles before entering capillary networks.

• 1. Capillary Protection: Capillaries have very thin walls that are fragile. The lower pressure prevents them from being damaged or bursting.
• 2. Efficient Exchange: The reduced pressure speeds up blood flow through the capillaries, allowing sufficient time for effective exchange.
• 3. Fluid Balance: Lower pressure towards the venous end of the capillary bed facilitates the reabsorption of tissue fluid back into the capillaries.

Compare and contrast the typical histological structure of a muscular artery with that of an arteriole, referencing differences in overall diameter, wall thickness relative to lumen size, and the composition (especially smooth muscle and elastic tissue) of their layers (tunica intima, media, and externa).

• 1. Similarities: Both have an innermost lining of endothelial cells (tunica intima); Both have a layer of smooth muscle (tunica media).
• 2. Differences (Muscular Artery vs. Arteriole): Overall Size & Wall Thickness: Muscular arteries are smaller in diameter and have thinner walls relative to their lumen size compared to arterioles.
• 3. Lumen: Muscular arteries generally have a wider lumen than arterioles (though arterioles can constrict significantly).
• 4. Tunica Media: Muscular arteries have a much thicker tunica media with more layers of smooth muscle.
• 5. Elastic Tissue: Muscular arteries have prominent internal and external elastic laminae and more elastic fibres within the tunica media compared to arterioles, which have less elastic tissue.
• 6. Tunica Externa: Muscular arteries have a thicker, more defined outer layer of connective tissue (tunica externa/adventitia) than arterioles.

For each of the following pathogens, state the major disease it causes and its typical mode(s) of transmission: Plasmodium species, Vibrio cholerae, Mycobacterium tuberculosis, and Human Immunodeficiency Virus (HIV). Also, indicate whether each pathogen is a prokaryote, eukaryote, or virus.

• 1. HIV: Virus; Disease: HIV/AIDS; Transmission: Body fluids.
• 2. Plasmodium species: Eukaryote (Protist); Disease: Malaria; Transmission: Faecal-oral route.
• 3. Vibrio cholerae: Prokaryote (Bacterium); Disease: Cholera; Transmission: Faecal-oral route.
• 4. Mycobacterium tuberculosis: Prokaryote (Bacterium); Disease: Tuberculosis; Transmission: Airborne droplets / Aerosol infection.

Tenofovir is an antiretroviral drug effective against HIV. After phosphorylation within the host cell, its structure resembles deoxyadenosine monophosphate but crucially lacks the 3′-hydroxyl (-OH) group normally present on the deoxyribose sugar. Explain the mechanism by which tenofovir inhibits the action of HIV’s reverse transcriptase enzyme.

• 1. Competitive Inhibition: Tenofovir (phosphorylated) structurally resembles the natural substrate (dATP) and competes for the active site of HIV reverse transcriptase.
• 2. Non-Competitive Inhibition: Tenofovir binds to an allosteric site on the enzyme, changing the active site shape.
• 3. Chain Termination: When incorporated into the growing viral DNA strand, synthesis halts because tenofovir lacks the necessary 3′-hydroxyl (-OH) group required for forming the phosphodiester bond with the next nucleotide, thus preventing chain elongation.

In 2019, approximately 605,000 people globally received Pre-Exposure Prophylaxis (PrEP) for HIV prevention. A United Nations target set in 2016 aimed for 3,000,000 people to receive PrEP. Calculate the number of people who received PrEP in 2019 as a percentage of this UN target. Provide your answer rounded to the nearest whole number.

• 1. Calculation: (605,000 / 3,000,000) * 100% = 20.166…%
• 2. Answer (nearest whole number): 21%

Describe four distinct public health strategies that authorities can implement to reduce the transmission rates of HIV, explaining the mechanism by which each strategy contributes to prevention.

• 1. Promote Condom Use: Provide/educate on correct use. Explanation: Physical barrier preventing infected fluid exchange.
• 2. Needle Exchange Programs: Provide sterile needles to IV drug users. Explanation: Reduces sharing of contaminated needles.
• 3. Screening Blood Donations: Test donated blood/products. Explanation: Prevents transmission via transfusions.
• 4. Antibiotic Therapy: Treat people with HIV using antibiotics. Explanation: Kills the virus directly.

Identify the two main specialized cell types forming the epithelial lining of the larger airways (e.g., trachea, bronchi) in the mammalian gas exchange system, noting their key structural features (e.g., surface projections, secretory function).

• 1. Cell with projections: Ciliated epithelial cell.
• 2. Secretory cell: Goblet cell.
• 3. Structural cell: Fibroblast.

Describe how the structure and coordinated action of the cells forming the pseudostratified ciliated columnar epithelium, including mucus production, contribute to the protection of the lungs from inhaled particles and pathogens.

• 1. Mucus Secretion: Goblet cells produce and secrete mucus.
• 2. Trapping Debris: Sticky mucus traps inhaled particles (dust, pathogens).
• 3. Mucociliary Escalator: Cilia on ciliated epithelial cells beat downwards, propelling mucus deeper into the lungs for disposal.

Describe the characteristic internal arrangement of microtubules, often referred to as the “9 + 2” array, that is observed in a transmission electron micrograph showing a cross-section of a eukaryotic cilium or flagellum.

• 1. The cross-section shows: Nine pairs (doublets) of microtubules arranged in a peripheral ring.
• 2. There are nine single microtubules located in the center.
• 3. Associated proteins like dynein arms may also be mentioned/visible.

Explain which structural feature definitively shows that a cilium, despite projecting outwards, is fundamentally an extension of the cell’s cytoplasm and therefore an intracellular structure.

• 1. The entire “9 + 2” microtubule structure of the cilium is enclosed within an extension of the cell surface membrane.
• 2. This shows it’s an outgrowth containing cytoplasm.
• 3. The cilium is attached directly to the nuclear envelope.

Describe the primary functions of centrioles within an animal cell. Explain their specific roles and behavior during the different phases of the mitotic cell cycle, such as in a dividing stem cell.

• 1. Function: Component of the centrosome (main Microtubule Organising Centre – MTOC); organise spindle fibre formation during cell division; involved in forming cilia/flagella.
• 2. Interphase (S/G2): Centriole pair replicates (two centrosomes formed).
• 3. Prophase: Centrosomes migrate to opposite poles.
• 4. Spindle Formation: Microtubules grow from centrosomes to form the mitotic spindle.
• 5. Chromosome Attachment & Separation: Spindle fibres attach to centromeres, align chromosomes (Metaphase), and separate sister chromatids (Anaphase).
• 6. Centrioles directly pull chromatids apart.

The rate of transpiration was measured for Helianthus annuus (sunflower) and Nerium oleander (oleander) under varying Leaf Vapour Pressure Deficit (LVPD) conditions (0 to 4.0 kPa). Based on typical responses (sunflower = mesophyte, oleander = xerophyte), compare the likely transpiration responses of these two species to increasing LVPD.

• 1. Both start at/near 0 rate at 0 kPa LVPD.
• 2. Both likely increase rate as LVPD increases initially.
• 3. H. annuus (sunflower) likely has a consistently lower rate than N. oleander (oleander) (except potentially at 0 LVPD).
• 4. H. annuus likely shows a steeper initial increase (more sensitive to moderate LVPD increases).
• 5. H. annuus might plateau or decrease rate at high LVPD due to stomatal closure, while N. oleander’s rate, though lower overall, might be maintained better at high LVPD due to adaptations.
• 6. A significant quantitative difference is expected, with H. annuus having a higher peak rate.

Nerium oleander is a xerophyte, adapted to dry conditions. Describe two distinct structural adaptations commonly found in its leaves that serve to minimize water loss via transpiration, and explain the mechanism by which each adaptation reduces the rate of water vapour diffusion from the leaf.

• 1. Adaptation: Stomata located in sunken pits/crypts. Explanation: Traps humid air, reducing water potential gradient, lowers transpiration. Protects from wind.
• 2. Adaptation: Thin, single-layered epidermis. Explanation: Allows rapid gas exchange but increases water loss.
• 3. Adaptation: Hairs (trichomes) often present within stomatal pits. Explanation: Further traps moist air, reduces air movement, maintains humidity, lowering water potential gradient, reduces transpiration.
• 4. Adaptation: Thick waxy cuticle on the epidermis. Explanation: Impermeable barrier reducing water loss directly through epidermal cells (cuticular transpiration).

Explain how the specific sequence of amino acids in the variable regions of an antibody’s heavy and light chains leads to the formation of a unique three-dimensional antigen-binding site, conferring specificity for a particular epitope.

• 1. Unique Amino Acid Sequence: Variable regions have unique primary structure (amino acid sequence).
• 2. Specific 3D Shape: This amino acid sequence dictates the folding into a precise 3D tertiary/quaternary structure for the antigen-binding site.
• 3. Non-Complementarity: This unique 3D shape is generally non-complementary to the specific shape of an antigen’s epitope, allowing broad binding.

State the primary function attributed to the flexible hinge region present in many types of antibody molecules (such as IgG).

• 1. Provides rigidity, holding the antigen-binding “arms” in a fixed position.
• 2. Allows the two antigen-binding “arms” (Fab regions) to move relative to each other.
• 3. Enables binding to antigens/epitopes spaced differently or located on curved surfaces.

Explain the immunological advantage provided when antibodies, bound to a pathogen or antigen, also bind via their constant region to specific receptors present on the surface of phagocytic cells like macrophages (a process known as opsonisation).

• 1. This process (opsonisation) flags or tags the pathogen/complex for phagocytosis.
• 2. It makes it much easier for the macrophage (phagocyte) to recognise, engulf, and destroy the antibody-coated target.
• 3. It inhibits phagocytosis efficiency.

Describe the post-transcriptional process of alternative splicing as it occurs in differentiating B lymphocytes (plasma cells). Explain how this mechanism allows for the generation of a vast diversity of antibody variable regions from a finite number of immunoglobulin gene segments encoded in the genome. (Note: While V(D)J recombination is the primary driver of variable region diversity *before* transcription, alternative splicing can affect constant regions and membrane-bound vs secreted forms, but the question frames it regarding variable regions – interpret broadly or focus on the general splicing concept).

• 1. After transcription, the primary RNA transcript contains coding regions (exons) and non-coding regions (introns).
• 2. Intron Removal: Non-coding introns are spliced out (removed) from the primary transcript.
• 3. Exon Recombination (Alternative Splicing): The remaining exons can be joined together in different combinations to produce different mature mRNA molecules.
• 4. All potential exons present in the primary transcript must always be included in the final mature mRNA.
• 5. This process allows a single gene (or pre-mRNA) to potentially code for multiple different protein isoforms (like different antibody forms or other proteins).